Multi-string LED drive system

摘要

A drive system for multiple LED strings powered by a common line voltage. Current control circuits are connected in series with respective LED strings; each current control circuit includes a drive transistor (typically a FET) which causes a desired LED string current to be conducted. In one embodiment, the current conducted by a selected one of the LED strings is controlled by the line voltage regulation loop, while the currents conducted by the remaining LED strings are controlled by respective local current loops, thus avoiding conflicts between the local current and line voltage regulation loops. The LED string to be current regulated by the line voltage regulation loop can be determined by a variety of criteria, such as the current control circuit having the maximum FET gate voltage, the minimum FET source voltage, the minimum FET drain voltage, the maximum FET gate-source voltage, or the minimum FET drain-source voltage.

主权项

1. A multi-string LED drive system for controlling the currents conducted by two or more LED strings which are powered by a common line voltage, said system comprising:
a plurality of current control circuits connected in series with respective LED strings, at least some of said current control circuits receiving respective reference signals and configured to operate as respective local current loops which regulate the currents conducted by their respective LED strings to values which vary with said received reference signals, wherein each current control circuit includes a current control transistor and an amplifier; a switching array operable to select any one of said current control circuits as said selected one of said current control circuits; and a line voltage regulation circuit configured to operate as a line voltage regulation loop which receives a maximum gate voltage of current control transistors and a target signal at respective inputs, and is arranged to generate said common line voltage such that said maximum gate voltage of current control transistors is approximately equal to said target signal, said selected one of said current control circuits configured such that the current conducted by the LED string connected to said selected current control circuit is regulated to a value that varies with said target signal by said line voltage regulation loop, wherein for each of said current control circuits, said switching array comprises: a first switch which connects the control input of said transistor to a fixed voltage when closed thereby driving said transistor to fixed voltage; a second switch which connects the output of said amplifier to the control input of said transistor when closed; and a third switch which connects said junction to said line voltage regulation circuit when closed; such that said first and third switches are open and said second switch is closed when said current control circuit regulates its LED string current with a local current loop, and such that said first and third switches are closed and said second switch is open when said current control circuit operates as said selected current control circuit and therefore regulates its LED string current with said line voltage regulation loop.